Simply just adding a border of a different colour to a shape in edit mode
I see at least 3 solutions. Note that if you want no shadow on your yellow, use an Emission shader:

1/ Create edge loops close to your angles, and assign your yellow color to all these edges.

2/ Duplicate your cube and give it a Wireframe modifier. Play with the parameters to increase or decrease the thickness of the wireframe.

3/ In Photoshop, create a picture with a blue square and yellow outlines. In the UV/Image Editor of Blender, open this picture, unwrap your cube and give it a material with an Image Texture node, with the cube picture as the image.